The surfaces in the hot gas area are provided nearly completely with coatings in modern gas turbines. The heat insulation layers used in such applications are used to lower the material temperature of cooled components. As a result, the service life can be prolonged, the cooling air can be reduced, or the machine can be operated at higher inlet temperatures. Heat insulation systems always comprise a metallic adhesive layer connected with the base material (base metal) by diffusion and a superjacent ceramic layer with poor thermal conductivity, which is the actual barrier against the heat flow and protects the base metal against high-temperature corrosion and high-temperature erosion.
Zirconium oxide, which is partially stabilized with about 7 wt. % of yttrium oxide (international acronym “YPSZ” from Yttria Partially Stabilized Zirconia), has proved to be a suitable ceramic material for the heat insulation layer. The heat insulation layers are classified to two essential classes according to the particular method employed to apply them. Depending on the desired layer thickness and the stress distribution, a porosity between about 10 vol. % and 25 vol. % is set in the case of the thermally sprayed layers (mostly layers sprayed with atmospheric plasma, APS). The binding to the rough-sprayed adhesive layer is brought about by mechanical clamping.
Heat insulation layers that are applied by vapor deposition carried out by physical vapor deposition processes by means of an electron beam (EB-PVD processes) have a columnar, stretching-tolerant structure if certain deposition conditions are complied with. The layer is bound chemically in the case of this process due to the formation of an Al/Zr mixed oxide on a pure aluminum oxide layer (Thermally Grown Oxide, TGO), which is formed by the adhesive layer during the application and subsequently during the operation. This process imposes special requirements on the oxide growth on the adhesive layer. In principle, both diffusion layers and support layers may be used as adhesive layers.
The following complex requirements are imposed on the adhesive layers, namely, low static and cyclic rates of oxidation, formation of the purest possible aluminum oxide layer as a TGO (in case of layers prepared according to the EB-PVD process), sufficient resistance to high-temperature corrosion, low brittle/ductile transition temperature, high creep strength, good adhesion, minimal long-term interdiffusion with the base material, and economical application of the adhesive layer with a reproducible quality.
Metallic support layers from a special alloy based on MCrAlY (M=Ni, Co) offer the best possibilities for meeting the chemical and mechanical requirements for the special requirements imposed in stationary gas turbines. The properties of the support layers can be further improved by the addition of special refractory alloying elements such as rhenium and tantalum or by alitizing. MCrAlY layers contain the intermetallic β phase NiCoAl as an aluminum reserve in an NiCoCr (“γ”) matrix. However, this phase also has an embrittling effect, so that the Al content that can be reached in practice in the MCrAlY layer is less than 12 wt. %. To further increase the oxidation resistance, it is known (WO 96/34129) that the MCrAlY layers can be coated with an Al diffusion layer in order to increase the Al content of these layers. However, this process has hitherto been extensively limited to low-aluminum starting layers because of the risk of embrittlement.